The typical negative feedback circuit for an amplifier uses only resistive elements. A resistor dissipates power, thus losing a significant amount of power and resulting in lower power being delivered to the amplifier load. Operation of the amplifier at high power is precluded since the feedback signal must also have high power, but large amounts of powr are dissipated by the feedback network. The typical amplifier thus cannot achieve an optimum level of distortion.
A resistor also introduces noise into a circuit. Thus, an amplifier with a resistive feedback network suffers from the noise introduced by the resistors.
The feedback network is typically designed to control gain and the input and output impedances of the amplifier. This feedback networks employ resistive elements and also affect the power efficiency and signal-to-noise ratio of the amplifier.
It has been suggested to avoid the disadvantages inherent in resistive feedback networks by using elements in the feedback network which do not dissiplate power. Ernst H. Nordholt, in an article entitled "Classes and Properties of Multiloop Negative-Feedback Amplifiers"; IEEE Transactions on Circuits and Systems, March, 1981, calls these elements "non-energic." The ideal gyrator and the ideal transformer are cited as examples of such non-energic elements, and Nordholt describes a number of amplifier configurations which employ non-energic elements in the feedback network. For example, FIG. 6 of the Nordholt article shows a configuration using two transformers for the feedback elements, wherein both of the transformers are connected to the inverting input of an amplifier.
It has also been suggested to use a directional coupler in the feedback network. U.S. Pat. No. 3,624,536 (Norton) describes an amplifier which utilizes a directional coupler to provide the feedback signal. The Norton directional coupler is a four-port device wherein the individual ports of two sets of ports (a-c and b-d) are isolated from each other. Ports (a-b) are coupled and are in-phase, and ports (a-d) are coupled 180 degrees out of phase. Ports (b-c) are coupled in phase. The input signal is applied to port (a), and port (b) is connected to the input of the amplifier. Port (c) is connected to the output of the amplifier and port (d) is the output of the device.
The majority of the feedback in the Norton circuit occurs through the connection of port (c) to port (b) without phase shift.
U.S. Pat. No. 4,042,887 (Meade, et al.) discloses an amplifier also using a plurality of directional couplers to provide negative feedback to the amplifier.